The flexibility of communication networks based on light signals conducted via optical fibres is greatly increased by the availability of optical fibre-compatible optical components such as optical circulators. Optical circulators enable light signals to be routed from one optical fibre to another, and prevent reflected light from returning to the source of the light.
Conventional optical circulators are made of discrete optical elements. As a result, such optical components are bulky and expensive. A substantial portion of the cost of making such optical components arises because the elements of such components are physically large and have to be individually and precisely aligned relative to one another. For example, a recent catalog published by the Fuji Electrochemical Corporation, Tokyo, Japan shows a Model YC-125A three-port optical circulator with dimensions of 43.times.30.times.8 mm, excluding the optical fibre connectors. The large size of these components limits the density with which optical fibre switching systems can be built.
In High-Isolation Polarization-Insensitive Optical Circulator for Advanced Optical Communication, 10 J. LIGHTWAVE TECHNOLOGY, 1210-1217 (1992, September) M. Koga and T. Matsumoto describe an optical circulator based on birefringent crystals, Faraday rotators and optically-active rotators. However, in the Koga and Matsumoto optical circulator, the lateral spacing between the input ports is not precisely defined, so the optical circulator divides each light beam into orthogonal polarization components laterally spaced from one another by about 1.2 mm. This requires the use of birefringent crystals some 12 mm thick. Also, the Koga and Matsumoto optical circulator uses optically-active rotators about 15.8 mm thick to rotate the direction of polarization of the polarization components. These large components cause the Koga and Matsumoto optical circulator to be physically large, and make it unsuitable for use in applications in which small size and low cost are important.
In U.S. Pat. No. 5,734,763, assigned to the assignee of the present application, the inventor disclosed a number of different optical components, including optical circulators, based on birefringent walk-off crystals. The linear dimensions of the elements of the optical components disclosed in this patent application are less than one-tenth of those of the elements of conventional optical components, which resulted in these optical components being substantially smaller and lower in cost than conventional optical components.
In the optical circulators disclosed in the inventor's prior application, the fraction of the light beam received at PORT 1 transmitted to PORT 1' is proportional to cos.sup.2 .DELTA..theta..sub.F and the fraction of the light beam received at PORT 2 transmitted to PORT 1', i.e., the crosstalk between PORT 2 and PORT 1', is proportional to sin.sup.2 .DELTA..theta..sub.F, where .DELTA..theta..sub.F is the rotational alignment error between the walk-off directions of the opposed walk-off crystal pair of the first I/O port and the walk-off directions of the opposed walk-off crystal pair of the second I/O port. To achieve a high transmission factor and low crosstalk requires an accurate rotational alignment between the first and second I/O ports. This increases the manufacturing cost of such optical circulators.
To further increase the flexibility of communication networks based on light signals conducted via optical fibres, it is desirable to increase the density with which optical fibre switching systems can be built, and to reduce substantially the cost of the optical components used in such switching systems. Accordingly, it would be advantageous to have a three-port optical circulator that not only has the considerably smaller size and reduced material cost of the inventor's previously-disclosed optical circulators, but that is also more tolerant of manufacturing alignment errors.